1. Field of the Invention
The present invention relates to compositions which can be used to apply conductors to electronic components such as printed circuit boards and semiconductors, particularly, to compositions which can be applied and converted to solid conductors at temperatures below 450xc2x0 C.
2. Related Art
A common method for printed circuit fabrication process is subtractive or semi-additive processes in which conductors are formed by etching away unwanted copper. A fully additive process would have many advantages over the subtractive or semi-additive methods. The primary problem in providing a wholly additive process for producing printed circuitry is the requirement for high electrical conductivity with low enough curing temperature to be compatible with the polymer-based circuit boards. Another major problem is making connections to the additive traces, preferably by conventional soldering. Present technology includes low cure temperature conductive epoxies and transient liquid phase materials which produce traces with poor electrical conductivity and poor solderability or high temperature thick film inks which produce traces with good electrical conductivity and good solderability but which are limited to ceramic substrates. These small, expensive and specialized substrates are required to withstand the thick film ink firing temperatures of more than 650xc2x0 C. and usually above 850xc2x0 C. A method which could duplicate the performance of thick film inks but on polymer-based substrates at 250 to 350xc2x0 C. would permit application of this technology broadly in the $27 billion rigid circuit board industry and the $2.5 billion flexible circuit industry, worldwide.
xe2x80x9cThick filmxe2x80x9d technology is routinely practiced to produce hybrid circuits on ceramic substrates. R. W. Vest, xe2x80x9cElectronic Ceramicsxe2x80x9d, R. Breckenridge, ed., 1991. The conductor patterns are created by silk screening or stencil printing thick film pastes or inks onto ceramic substrates and firing them at temperatures of 850 to 1100xc2x0 C. to reduce the metal-containing inks to metal. An example of such inks are silver-palladium compositions which have recently been reviewed by Wang, Dougherty, Huebner and Pepin, J. Am. Ceram. Soc. 77(12), 3051-72 (1994). Typically thick film inks contain metal powders, an inorganic glass binder and a vehicle consisting of. a polymer binder and a solvent. The vehicle provides the correct consistency for screen printing and consists typically of a polymer such as ethyl cellulose, hydrogenated rosin or polyacrylics dissolved in a low volatility solvent. Common solvents are terpineol, dibutyl carbitol and various glycol ethers and esters. The inks are applied to ceramic substrates by screen printing, dried to drive off the solvent and heat treated, usually in a belt furnace, to decompose the polymer binder and fuse the metal and the inorganic glass binder. The glass phase provides the bond to the substrate which is usually alumina, and the metal provides the electrical conductivity. Typically the conductors have a striated cross section with layers of glass alternating with layers of metal. The glass tends to concentrate at the ceramic interface and the metal at the air interface. The conductivity is typically one half to one quarter that of the bulk metal.
A number of thick film compositions contain surfactants to improve screenability and stability of the metal powder dispersions. Often these surfactants are metallo-organic compounds such as soaps of carboxylic acids. These are convenient in that they will decompose at relatively low temperature to deposit the metal or its oxide which can perform a useful function in the fired conductor.
U.S. Pat. No. 5,071,826 issued on Dec. 10, 1991 and U.S. Pat. No. 5,338,507 issued on Aug. 16, 1994 to J. T. Anderson, V. K. Nagesdh and R. C. Ruby, disclose the addition of silver neodecanoate to superconducting oxide mixtures in which the neodecanoate is decomposed to the metal at 300xc2x0 C. to coat the superconducting grains with silver. The coated grains are then sintered and oxidized at 600-800xc2x0 C. to produce an oxide superconductor of enhanced strength and critical current.
The addition of titanate to thick film conductors by decomposition of an organo-metallic titanate is described by K. M. Nair in U.S. Pat. No. 4,381,945 issued on May 3, 1983.
U.S. Pat. No. 4,599,277 issued on Jul. 8, 1986 to J. M. Brownlow which discloses adding organo-metallic compounds to thick film inks to increase the densification temperature of the metal to match that of the ceramic substrate at 850-950xc2x0 C., the inverse of the process required to apply conductors to polymer circuits at low temperatures.
Conventional thick film paste compositions containing silver flake, glass flit and silver resinates, which are carboxylic acid soaps, as well as surfactants such as Triton X 100, were described in U.S. Pat. No. 5,075,262, issued on Dec. 24, 1991 and U.S. Pat. No. 5, 183,784, issued on Feb. 2, 1993 to M. N. Nguyen and coworkers. The objective was to eliminate the preliminary drying step after printing, and the resinate was said to promote adhesion and minimize cracks and voids in bonding semiconductor dies to a ceramic substrate at 350-450xc2x0 C.
V. K. Nagesh and R. M. Fulrath were issued U.S. Pat. No. 4,130,671 on Dec. 19, 1978 which was assigned to U.S. Department of Energy. It discloses a similar composition of glass frit and silver resinate which was decomposed at low temperature to provide silver-coated glass particles similar to the superconductor of Anderson above. The particles were applied to a substrate either before or after decomposition of the resinate and fired in an oxidizing atmosphere at 500 to 700xc2x0 C. to provide a conductor of metal-coated glass particles.
Still other conventional thick film compositions of glass and metal powders in an organic vehicle but without the resinate are described in U.S. Pat. Nos. 5, 250,229 and 5,378,408.
To create a low temperature analog of the thick film process, it will be necessary to find a new mechanism to obtain adhesion and cohesion of the deposited metal which can operate at temperatures below 450xc2x0 C., which is the extreme upper temperature limit that most polymers can tolerate. The use of inorganic glass powder binders which are universally used in conventional thick film inks is not possible in this application because none of them melt a low enough temperature, and the glass will not bond to the metal or to the polymer substrates.
Other approaches to this objective have been described. The most common one is the creation of electrically conductive inks or pastes by incorporating metal powder, usually silver powder, in an organic matrix, the so-called xe2x80x9cPolymer Thick Film xe2x80x9d materials. This is a major industry with products available from Ablestik, AIT, Hokurika, M-Tech, Thermoset, Epoxy Technology and Ferro, among others. These materials can be printed on circuit boards, and they have good adhesion. An example of the application of this technology was described in an article by K. Dreyfack in Electronics 52(17), 2E-4E, 1979, on Societie des Produits Industrielles ITT""s silk screening silver and graphite-based conductors of this type onto rigid and flexible circuits. One problem with this approach is that the inks conduct by random contacts between powder grains in the organic matrix, and the conductivity is poor. Typical values of the resistivity, which is the reciprocal of conductivity, are 40 to 60 microohm cm, compared to bulk silver at 1.59 microohm cm and high temperature thick film conductors at 3-6 microohm cm. Still more disturbing is the fact that the electrical conductivity is not constant with time. The conductivity depends on adventitious contacts between individual metal grains which are prone to be made and broken randomly as the trace is heated and cooled and particularly as it is exposed to moisture and other environmental influences. Another major problem with polymer thick film materials is that because of their organic content, they are not solderable.
A typical resin-bonded copper powder conductor is described in Japanese Patent Application 52-68507, June, 1977. U.S. Pat. No. 4,775,439 issued on Oct. 4, 1988 to R. E. Seeger and N. H. Morgan, describes a more elaborate polymer thick film approach. In this concept metal powder and binder are applied to a substrate and dried. The trace is then covered by a polymer film which is adhesively laminated to the substrate to hold the conductor in place. This does not address the problem of obtaining electrical conductivity comparable to bulk metal.
Bulk conductivity has been achieved at low temperature by decomposing metallo-organic compounds on various substrates. They can be applied by ink jet printing as described by R. W. Vest, E. P. Tweedell and R. C. Buchanan, Int. J. of Hybrid Microelectronics 6, 261-267, 1983. Vest et al have investigated so-called MOD (Metallo-Organic Decomposition) technology over many years. The most relevant aspect of this research was reviewed in xe2x80x9cLiquid Ink Jet Printing with MOD Inks for Hybrid Microcircuitsxe2x80x9d Teng, K. F., and Vest, R. W., IEEE Transactions on Components, Hybrids and Manufacturing Technology, 12(4), 545-549, 1987. The authors described their work on printing silver and gold conductors as well as dielectrics and resistors. MOD compounds are pure synthetic metallo-organic compounds which decompose cleanly at low temperature to precipitate the metal as the metallic element or the oxide, depending on the metal and the atmosphere. The noble metals, silver, gold and the platinum group decompose to metal films in air. The organic moiety is bonded to the metal through a hetero-atom providing a weak link that provides for easy decomposition at low temperature. An oxygen bond, as in carboxylic acid-metal soaps, has been found to be satisfactory, as have amine bonds for gold and platinum.
Vest et al investigated metallization of ceramic substrates and silicon by ink jet printing of xylene solutions of soaps such as silver neodecanoate and gold amine 2-ethylhexanoate. Images of satisfactory resolution (0.003 inches or 75 microns) were obtained, but the conductivity was low because of the extremely small thickness of the layers after decomposition which was less than a micron. Preliminary experiments by Partnerships Limited on epoxy-glass circuit boards with silver neodecanoate solutions demonstrated that well-bonded conductors could be produced on polymer substrates. Again, the difficulty was that they were very thin and had inadequate conductivity. It was found that the addition of more MOD compound resulted in wider traces but not thicker ones. The MOD compound melts before decomposing and spreads over the surface uncontrollably. Since melting provides for a well-consolidated metal deposit after decomposition, which is desirable, and since some MOD compounds are actually liquids at room temperature, this is an unavoidable problem. A possible solution to this problem is to build up the thickness by printing many layers, which Vest et al found suitable for metallizing silicon solar cells, but this detracts from the single pass production of circuits, which is our objective.
Similar materials and techniques have been used to apply thin film metallization and seed coatings which are then built up with solder or electroplating. U.S. Pat. No. 4,650,108, issued on Mar. 17, 1987, to B. D. Gallegher and assigned to the National Aeronautics and Space Administration, Washington, D.C.; U.S. Pat. No. 4,808,274 issued on Feb. 28, 1989, to P. H. Nguyen; U.S. Pat. No. 5,059,242 issued on Oct. 22, 1991 to M. G. Firmstone and A. Lindley and U.S. Pat. No. 5,173,330 issued on Dec. 22, 1992, to T. Asano, S. Mizuguchi and T. Isikawa, are examples. Thin films alone cannot provide adequate conductivity.
A creative attempt to circumvent the resistivity problem was described in U.S. Pat. No. 4,487,811 issued on Dec. 11, 1984, to C.W. Eichelberger. The patent describes augmenting the conductivity by a replacement reaction of metal in the deposit by a more noble metal in solution, for example the replacement of iron by copper. In the process of doing this, the contact between particles is improved by the greater volume of the replacement metal and its greater intrinsic conductivity. A resistivity of 7.5 microohm cm was achieved, substantially better than silver-loaded epoxies, but short of the performance of thick film inks.
The replacement reaction solved yet another problem of polymer inks in that the material was solderable, which conductive epoxy formulations in general are not. Another approach to solderability was described in U.S. Pat. No. 4,548,879 issued on Oct. 22, 1985 to F. St. John and W. Martin. Nickel powder was coated with saturated monocarboxylic acid with ten or more carbon atoms. The coated powder was mixed with novolac epoxy resins in a butyl carbitol acetate vehicle and silk screened onto an epoxy-glass board. After curing at 165xc2x0 C., the conductive trace could be solder-coated by fluxing and dipping into molten solder, while a trace made with uncoated nickel powder could not be soldered. No improvement in electrical conductivity was described with this process.
A silver powder is disclosed in xe2x80x9cNovel Silver Powder Compositionxe2x80x9d, U.S. Pat. No. 4,186,244 issued Jan. 29, 1980, and xe2x80x9cProcess for Forming Novel Silver Powder Compositionxe2x80x9d, U.S. Pat. No. 4,463,030 issued Jul., 31, 1984, Both issued to R. J. Deffeyes, and H. W. Armstrong and assigned to Graham Magnetics, Inc. North Richland Hills, Tex. The silver powder was formed by decomposing dry silver oxalate in the presence of a long chain carboxylic acid, either saturated (stearic acid, palmitic acid) or unsaturated (oleic acid, linoleic acid). The acid reacted with the metal powder as it was formed to provide a protective coating on the surface and to limit the particles to sub-micron size. The particles were washed to remove excess acid and blended with an equal weight of a conventional thick film vehicle consisting of ethyl cellulose polymer binder and pine oil solvent.
The resulting ink was coated on a ceramic or polyimide substrate and heated to 250xc2x0 C. in air for 30-90 seconds to convert the coated powder to a silver conductor with a stated conductivity of one ohm per square, which is not adequate for practical circuitry with traces many hundreds or thousands of squares long. The coating is said to be solderable without flux, which is believable if residual acid is acting as a flux. It is stated to be resistant to leaching in a bath of molten solder, which is unexpected, based on the well known solubility of silver in solder.
A somewhat similar silver flake material was patented by Grundy of Johnson and Matthey, U.S. Pat. No. 4,859,241, Aug. 22, 1989. The flake was prepared by milling silver powder with silver stearate surfactant in an organic solvent to produce silver stearate-coated silver flakes providing a glass-filled ink composition of superior stability. This is a common method of preparing stable powders and flakes of silver.
Another class of materials used to produce additive electronic circuitry are the Transient Liquid Phase materials developed by Toronaga Technologies under the trade name xe2x80x9cOrmetxe2x80x9d. These materials and their applications are described by P. Gandhi Circuit World 23 (1), October, 1996, p. 43-46, and Roberts, E.; Proceedings of NEPCON WEST ""96, 3, 1748-1752, 1996. The materials consist of a mixture of powdered silver or copper conductor with powdered solder and a polymer binder. They can be printed like conductive epoxies but when heated, the solder melts and alloys with the conductor creating a network of fused metal. Further heating at temperatures in the neighborhood of 220xc2x0 C. for 10 minutes cures the polymer binder which provides for adhesion of the conductor to the polymer substrate. An alternative is to provide an adhesive layer on the substrate as disclosed by M. A Capote and M. G. Todd of Toranaga Technologies in U.S. Pat. No. 5,538,789, Jul. 23, 1996 and U.S. Pat. No. 5,565,267, Oct. 15, 1996.
Typically Ormet compositions yield electrical resistivities in the range 20-30 microohm-cm. there is also an issue with solderability because of the presence of the polymer binder.
None of the materials or mixtures described above accomplish the goal of providing a composition which can be cured to a well-bonded, well-consolidated metallic conductor with an electrical conductivity comparable to conventional thick film inks but with a curing temperature below 350xc2x0 C., preferably below 300xc2x0 C., more preferably below 275 C., which is required for compatibility with conventional polymer-based circuit board substrates. None of these materials has made it possible to impact the circuit board industry with new technology for rapid production by a simple process with no hazardous waste production. A new approach to provide this low temperature capability is needed.
The present invention provides printable compositions and processes for applying them to temperature-sensitive substrates and curing them to traces of high electrical conductivity at temperatures which the substrates can withstand. The essential constituents of these compositions are 1) a metal powder mixture of specified characteristics and 2) a Reactive Organic Medium (ROM) in which the consolidation of the metal powder mixture to a solid conductor takes place.
The metal powder mixture is comprised of a mixture of at least two type of metal powders: 1) metal flakes with a preferred major diameter of approximately 5 micrometers and a thickness-to-diameter ratio of 10 or more and 2) colloidal or semi-colloidal metal powders with mean diameters less than about 100 nanometers which are not aggregated to any great degree.
The ROM can consist of any metallo-organic compound which is readily decomposable to the corresponding metal, or an organic compound which can react with the metal to produce such a compound. Examples are metal soaps and the corresponding fatty acids. Other examples are metal amines and metal mercapto compounds and their corresponding amino and sulfide precursors.
The constituents of these compositions are weighed out in appropriate proportions, mixed with additional surfactants or viscosity modifiers if needed to provide the proper consistency, and milled together as on a three roll mill to provide a homogeneous, printable composition.
The composition is printed on the substrate using any convenient printing technology. Screen printing and stenciling are suitable for rigid substrates in relatively small numbers with high resolution. Gravure printing, impression printing and offset printing are suitable for high production rates on flexible substrates. Ink jet printing and electrostatic printing offer the additional advantage of direct computer control of the printed image. This permits circuits to be printed directly from Computer Aided Design (CAD) files and eliminates the need for special tooling. Each circuit can be different, if desired, for coding or prototyping. The same end can be achieved at lower production rates with computer-controlled dispensing equipment. This equipment produces dots or lines by moving a needle over the surface and dispensing printing composition supplied by a pump or pressurized syringe.
Substrates to which these compositions can be applied include rigid, glass-reinforced epoxy laminates, polyimide films for flexible circuits, other polymer-based electronic components, metal pads and semiconductor components. The compositions adhere naturally to most epoxy surfaces. Good adhesion to polyimide films requires the presence of a coating. FEP TEFLON(copyright) and low glass transition point polyimide coatings have been found to be satisfactory.
Adhesion to metals requires a clean metal surface, similar to the requirements for soldering. Acid constituents in the ROM act as fluxes to promote adhesion. Plating or tinning the metal pads is also effective. The use of organic solder protectants on copper pads is effective. Adhesion to semiconductors requires metallization with which the compositions are compatible.
The compositions are cured by exposure to heat for a short period of time. This time varies with the temperature to which the substrate can safely be exposed, but is less than a minute to achieve most of the electrical conductivity of which the composition is capable, and in some cases is less than 10 seconds at temperature.
Silver and gold may be cured in air. Copper and other non-noble metals require a protective atmosphere. Nitrogen with less than about 3 parts per million of oxygen has been found suitable for processing copper compositions. Addition of water vapor during the curing process, but not before or after, has been found to be beneficial in curing copper compositions.
The compositions of the present invention can be selectively applied only where conductors are required on a temperature-sensitive substrate by any convenient printing technology. These include screen printing, stenciling, gravure printing, impression printing, offset printing, ink jet printing and electrostatic printing and copying. Unexpectedly, it has been found that when heated, these compositions cure in seconds to well-consolidated, well-bonded conductive traces of pure metals at temperatures hundreds of degrees lower than required for conventional metallurgical sintering processes. This provides a wholly new capability to create printed circuitry at higher speed and lower cost than with conventional technology. The hazardous waste production characteristic of conventional photolithography, plating and etching processes is completely eliminated.